Please help, quick and simple. Potential Energy of Spring?

Block A has mass 1.00 kg, and block B has mass 3.00 kg. The blocks are forced together, compressing a spring between them; then the system is released from rest on a level, frictionless surface. The spring, which has negligible mass, is not fastened to either block and drops to the surface after it has expanded. Block B acquires a speed of 1.10 m/s.

a) What is the final speed of block A? Found this answer to be 3.30 m/s

b) How much potential energy was stored in the compressed spring?
- how do you find this answer??

isn't that how to find the final speed of block A? or am i missing something...
becuase i already have that....
i have all of the information for this problem and just need to find the potential energy that is stored in the spring....